Through air drying has become the technology of preference for making tissue for many manufacturers who build new tissue machines as, on balance, through air drying ("TAD") offers many economic benefits as compared to the older technique of conventional wet-pressing ("CWP"). With through air drying, it is possible to produce a single ply tissue with good initial softness and bulk as it leaves the tissue machine.
In the older wet pressing method, to produce a premium quality tissue, it has normally been preferred to combine two thin plies by embossing them together. In this way, the rougher air-side surfaces of each ply may be joined to each other and thereby concealed within the sheet. However, embossing two plies together imposes marked economic penalties which can be avoided in production of a one-ply product using through air drying. But even though through air drying has been preferred for new machines, conventional wet pressing is not without its advantages as well. Water may normally be removed from a cellulosic web at lower energy cost by mechanical means such as by overall compaction than by drying using hot air. It is not normally economic to convert older CWP tissue machines to TAD. Further, single ply machines can normally run at high speeds.
What has been needed in the art is a method of making a premium quality or near premium quality single ply tissue using conventional wet pressing. In this way, advantages of each technology could be combined so older CWP machines can be used to produce high quality single ply tissue at costs which are far lower than those associated with embossing two plies together.
One of the more significant barriers to production of a single ply CWP tissue has been the extreme sidedness of single ply webs using technology known prior to this invention. TAD processes can produce a nice soft bulky sheet having fairly low strength and good similarity of the surface texture on the front of the sheet as compared to the back. Having the same texture on front and back is considered to be quite desirable in these products or, more precisely, having differing texture is generally considered quite undesirable.
We have found that we can produce a soft high strength CWP tissue with low sidedness by judicious combination of several techniques as described herein. Basically, these techniques fall into four categories: (i) fiber stratification; (ii) chemical stratification; (iii) low angle, high adhesion creping; and (iv) reverse embossing. Of these four techniques, the first two seem to be more flexible and exhibit more pronounced benefits than the latter two, but by various combinations of these techniques as described, taught and exemplified herein, it is possible to almost "dial in" the required degree of sidedness depending upon the desired goals.
CWP processes can be carried out on fourdrinier, twin wire, suction breast roll, and crescent forming machines. Energy consumption is lower and the production speeds can be considerably higher than those used on TAD machines. The plies previously produced on CWP machines are usually fairly strong but, as mentioned, they have a distinctly two-sided character; consequently, CWP is most commonly used for two-ply products so that the softer sides of each ply can be positioned on the exterior of each sheet and the harsher surfaces buried in the interior, each facing the other. However, there is a substantial cost penalty involved in the production of two-ply products because the parent rolls of each ply are not always of the same length, and a break in either of the single plies forces the operation to be shut down until it can be remedied. Further, CWP plies in a multi-ply structure need to be embossed to bond the plies together and help restore some of the bulk squeezed out in the pressing operation used to dewater each ply. For these reasons, many single-ply CWP products currently found in the marketplace are typically low end products. These products often are considered deficient in thickness, softness, and exhibit excessive two sidedness. Accordingly, these products have had rather low consumer acceptance and are typically used in "away from home" applications in which the person buying the tissue is not the user.